What is "Background" in "Background Extraction?"

[rodmichael]

I am quite new to AP and PI.  I am just learning both, probably not very well.  I have been imaging and processing the Western Veil Nebula and I keep running into the same question in my mind.  The question likely represents my own ignorance.

My sense of images of the Veil Nebula:  The Veil Nebula is a spherical shell of ionized gas and dust resulting from an explosion (Supernova) at a point source that probably has a diameter of 200 or so light-years, including expansion of 100 or so light years in our direction.  When we image the Veil we are therefore imaging through a spherical "wall" of largely doubly ionized Oxygen and Hydrogen alpha.  The "nebula" that we see in our images are the edges of the sphere where we are looking tangentially at dimensionally deeper parts of the shell of OIII and Ha (and other dust and gases).  However, the "background" must also consist, at least in part, of OIII and Ha made visible by our NB imaging.  In my images it appears that the "background" OIII and Ha fill the entire frame, because the shell has expanded also toward Earth and is therefore relatively larger in the 'X' and 'Y' axes than the apparent diameter of the edges of the Veil Nebula which is 100 light years more distant.

My Questions:
1.  Is my conceptual understanding correct? And, if it is,
2.  What is "Background" when it comes to other processes in PI, e.g., ABE, Background Neutralization, and Noise Reduction processes?
3.  How do I deal with this "background" which is, in reality, part of the Veil nebula in this case.  Do I subtract or divide or otherwise remove/reduce as noise this "background" to enhance the contrast of the "edges" of the sphere?

EDIT:  If I knew how, I'd be willing to upload a non-linear HOO-AIP *.xisf file for someone to experiment with.  But as we all know, this is a large file.

[RickS]

My Questions:
1.  Is my conceptual understanding correct? And, if it is,
2.  What is "Background" when it comes to other processes in PI, e.g., ABE, Background Neutralization, and Noise Reduction processes?
3.  How do I deal with this "background" which is, in reality, part of the Veil nebula in this case.  Do I subtract or divide or otherwise remove/reduce as noise this "background" to enhance the contrast of the "edges" of the sphere?

1. I mostly agree.  The Veil is a complex 3D object and there's going to be some narrowband flux across the whole face of the object that we see.  The part I didn't get was "the shell has expanded also toward Earth and is therefore relatively larger in the 'X' and 'Y' axes than the apparent diameter of the edges of the Veil Nebula which is 100 light years more distant."

2. ABE and DBE are normally used for removing light pollution gradients though they will remove real data too if you're not careful.  Background Neutralization is about making the "empty" areas of the image a neutral gray.  Noise reduction is applied more heavily to dim areas of an image because that's where the SNR is lowest (the fundamental noise source for us is shot noise which is the square root of the signal, so SNR is better in areas of high signal.)  So, the notion of background is a bit flexible.

3. If you have light pollution gradients then you'll want to remove them by subtraction.  This is difficult to do unless you have areas of the image that don't include signal from the object, i.e. a true background reference.  Division is only used to remove vignetting and hopefully you've already dealt with this with flat-fielding.  The big question is whether you treat the faintest signal areas as dark background and hide them or try to make them visible.  IMO,  that is an aesthetic consideration that depends on your tolerance for visible noise and perhaps your skill with noise reduction.

Hope that helps...

Out of interest, here's a somewhat gimmicky representation of the Veil in Ha and Oiii that shows quite extensive Ha: http://www.astrobin.com/165204/

Cheers,
Rick.

[rodmichael]

My Questions:
1.  Is my conceptual understanding correct? And, if it is,
2.  What is "Background" when it comes to other processes in PI, e.g., ABE, Background Neutralization, and Noise Reduction processes?
3.  How do I deal with this "background" which is, in reality, part of the Veil nebula in this case.  Do I subtract or divide or otherwise remove/reduce as noise this "background" to enhance the contrast of the "edges" of the sphere?

1. I mostly agree. . . The part I didn't get was "the shell has expanded also toward Earth and is therefore relatively larger in the 'X' and 'Y' axes than the apparent diameter of the edges of the Veil Nebula which is 100 light years more distant."...

I was merely trying to imagine the light cone with its focus at earth.  The sphere of The Veil shell of gases is expanding into the cone and at its greatest extent is perhaps 100 light years closer to Earth and fills the cone to a greater extent than does the light we see from the edges of the sphere that are 100 light-years more distant, basically at the same distance as the origins of X, Y, and Z.

I suspect my verbiage is as faulty as my conceptualization.

EDIT:  p.s., I like your animation.

[RickS]

I was merely trying to imagine the light cone with its focus at earth.  The sphere of The Veil shell of gases is expanding into the cone and at its greatest extent is perhaps 100 light years closer to Earth and fills the cone to a greater extent than does the light we see from the edges of the sphere that are 100 light-years more distant, basically at the same distance as the origins of X, Y, and Z.

Have a look at this diagram, which is not to scale 

EDIT:  p.s., I like your animation.

Thanks.

[drmikevt]

I also have sometimes struggled with 'What is the correct background?', esp in busy images.  Often, I will create 3-5 previews in different areas and run each through Background Neutralization to see which one yields the most neutral result.  Sometimes, the best results are found by taking those 3-5 previews and using the Preview Aggregator script to, well, aggregate them and use then use that resultant image as the background.

I don't know if this is overthinking or not, but sometimes the images are quite different in terms of neutral tone.

Mike

[RickS]

I think the new PhotometricColorCalibration tool coming in PI 1.8.5 will be great for colour calibrating difficult fields, at least for RGB data.